In electrography, an electrostatic charge image is formed on a dielectric surface, typically the surface of the photoconductive recording element or photoconductor. Development of this image is commonly achieved by contacting it with a dry, two-component developer comprising a mixture of pigmented resinous electrically insulative particles known as toner, and magnetically attractable particles, known as carrier. The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image. The toner particles are held on the surface of the relatively larger-sized carrier particles by the electric force generated by the friction of both particles as they impinge upon and contact one another during mixing interactions. During contact between the electrostatic image and the developer mixture, the toner particles are stripped away from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong attractive force of the electric field formed by the charge image which overcomes the bonding forces between the toner particles and the carrier particles. In this manner, the toner particles are attracted by the electrostatic forces associated with the charge image and deposited on the electrostatic image to render it visible.
It is known in the art to apply developer compositions of the above type to electrostatic images by means of a rotating-core magnetic applicator which comprises a cylindrical developing sleeve or shell of a non-magnetic material having a magnetic core positioned within. The core usually comprises a plurality of parallel magnetic strips which are arranged around the core surface to present alternative north and south magnetic fields. These fields project radially, through the sleeve, and serve to attract the developer composition to the sleeve's outer surface to form a brush nap, or what is commonly referred to in the art as, a "magnetic brush". It is essential that the magnetic core be rotated during use to cause the developer to advance from a supply sump to a position in which it contacts the electrostatic image to be developed. The cylindrical sleeve, or shell, may or may not also rotate. If the shell does rotate, it can do so either in the same direction as or in a different direction from the core. After development, the toner depleted carrier particles are returned to the sump for toner replenishment. The role of the carrier is two-fold: (a) to transport the toner particles from the toner sump to the magnetic brush, and (b) to charge the toner by triboelectrification to the desired polarity, i.e., a polarity reverse to that of the polarity of the charge of the electrostatic image on the photoconductive recording element or plate, and to charge the toner to the proper or desired degree (amount) of charge. The magnetic carrier particles, under the influence of the magnets in the core of the applicator, form fur-like hairs or chains extending from the developing sleeve or shell of the applicator. Since the charge polarity of the magnetic carrier is the same as that of the electrostatic image, the magnetic carrier is left on the developing sleeve of the applicator after the toner particles have been stripped away from the carrier during development of the electrostatic or charge image. Typically, a bias voltage is applied between the photosensitive material or plate and the developing sleeve of the magnetic applicator by means of an electric current externally applied to the developing sleeve or shell which flows through the magnetic brush. The purpose of the bias voltage primarily is to prevent, or at least substantially reduce, the occurrence of unwanted toner fogging or background development caused by the migration of a certain portion of the toner particles available for development from the carrier to a non-image area or portion of the photosensitive plate (or drum) during development due to an incomplete discharge of such non-image areas during exposure. Commonly referred to as background charge, these areas of incomplete discharge cause an attraction for and a migration of some of the available toner particles (particularly those toner particles possessing an insufficient quantity of charge) to the partially discharged areas during development which results in the development or coloration of areas of the electrostatic image pattern that should not be developed. Hence the term, "background development". The polarity of the bias voltage should be the same as the charge polarity of the photosensitive material. Namely, if the charge polarity of the photosensitive material or plate is positive, the positive polarity is selected for the bias voltage. Caution must be exercised in selecting the proper amount of bias voltage applied between the photosensitive material and the developing sleeve so that problems such as discharge breakdown are not caused in the photosensitive material or the magnetic brush or that toner migration of the toner particles from the carrier to the electrostatic image to be developed is not prevented due to the application of a disproportionate or excessive amount of bias voltage to the magnetic brush during development. Ordinarily, it is typical that the bias voltage be controlled to about 100 to 300 volts, particularly about 150 to 250 volts. This particular method of toner development is commonly referred to in the art as magnetic brush development.
Conventionally, carrier particles made of soft magnetic materials have been employed to carry and deliver the toner particles to the electrostatic image. U.S. Pat. Nos. 4,546,060 and 4,473,029, teach the use of hard magnetic materials as carrier particles and an apparatus for the development of electrostatic images utilizing such hard magnetic carrier particles, respectively. These patents require that the carrier particles comprise a hard magnetic material exhibiting a coercivity of at least 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/g when in an applied magnetic field of 1000 Oersteds. The terms "hard" and "soft" when referring to magnetic materials have the generally accepted meaning as indicated on page 18 of Introduction To Magnetic Materials by B. D. Cullity published by Addison-Wesley Publishing Company, 1972. As disclosed in aforementioned U.S. Pat. No. 4,546,060, when magnetic carrier particles which (a) contain a magnetic material exhibiting a coercivity of at least 300 Oersteds and (b) have an induced magnetic moment of at least 20 EMU/g when in an external magnetic field of 1000 Oersteds are exposed to a succession of magnetic fields emanating from the rotating core applicator, the particles interact with the moving fields to cause a turbulent rapid flow of developer as they flip or turn to move into magnetic alignment in each new field. Each flip, as a consequence of both the magnetic moment of the particles and the coercivity of the magnetic material, is accompanied by a rapid circumferential step by each particle in a direction opposite the movement of the rotating core. The observed effect is that the developer flows smoothly and at a rapid rate around the shell while the core rotates in the opposite direction resulting in a high level of triboelectrification of the toner while residing on the brush and the rapid delivery of fresh toner to the photoreceptor or photoconductive element thereby facilitating high-speed copying applications while providing for the complete development of electrostatic images at high-speed copying rates. In addition to providing development rates suitable for high-speed copying applications without the loss of image quality, the magnetic moment of the carrier particles is sufficient to prevent the carrier from transferring to the electrostatic image during development, i.e., there is provided sufficient magnetic attraction between the applicator and the carrier particles to hold the latter on the applicator shell during core rotation and thereby prevent the carrier from transferring to the image (i.e., carrier pick-up). These hard magnetic carrier materials represent a significant advancement in the art over the previously used soft magnetic carrier materials in that the speed of development is remarkably increased without experiencing a deterioration of the image. Speeds as high as four times the maximum speed utilized in the use of soft magnetic carrier particles have been demonstrated.
The above two mentioned U.S. patents, while generic to all hard magnetic materials having the properties set forth, prefer the hard magnetic ferrites which are compounds of barium and/or strontium such as, BaFe.sub.12 O.sub.19, SrFe.sub.12 O.sub.19, and the magnetic ferrites having the formula MO.6Fe.sub.2 O.sub.3, where M is barium, strontium or lead as disclosed in U.S. Pat. No. 3,716,630. While these hard ferrite carrier materials represent a substantial increase in the speed with which development can be conducted in an electrostatic apparatus, it has been found that development speed, i.e., development efficiency, progressively decreases in developer compositions comprising such hard ferrite magnetic carrier materials and oppositely charged toner particles as the particle size of the toner progressively decreases below about 8 micrometers. In addition, it has also been found that as the particle size of the toner progressively decreases below about 8 micrometers in such developer compositions, the density of the toned images produced thereby also decreases due to the inability of enough toner particles to be supplied to the development zone at a rate rapid enough to enable the complete development of the image. This is particularly noticeable in the solid, colored image area portions of the toner image which appear lighter or fainter in appearance than desired. This decrease in development or copying speed and toner image density is believed to be due primarily to the fact that the hard ferrite magnetic carrier particles of the prior art, such as those disclosed in U.S. Pat. No. 4,546,060, for example, depend solely upon triboelectrifcation or friction-charging of the toner particles as they impinge upon and intermix with the toner particles on the magnetic brush to attract the toner particles to the carrier particles and to adhere the toner particles to the carrier particle surface for transport to the development zone for development of the charge image. While friction-charging alone is sufficient to provide an adequate amount of toner particles to the development zone at a rate rapid enough to achieve the high development speeds and toner image densities referred to above when the toner particles used in the developer compositions along with the hard ferrite magnetic carrier particles have a particle size of approximately 8 micrometers or greater, friction-charging alone is not sufficient to provide such high development speeds and toner image densities when the particle size of the toner particles in such developer compositions falls below about 8 micrometers in diameter. This is believed to be due to the following. As the size of the toner particles used in the developer compositions progressively decreases below about 8 micrometers, the tendency of the individual toner particles in the toner supply sump to agglomerate or stick together and form clumps progressively increases due to the presence of very strong attractive surface forces among these very small-sized individual toner particles, such as those caused by Van der Vaals interactions, which cause a certain amount or portion of the individual toner particles to be attracted to one another and to form large clumps or agglomerates of toner particles. Since the surface areas provided by such agglomerates or clumps of toner particles which are available for tribo-charging by the carrier particles are much less than the surface areas of the individual toner particles that make-up the agglomerates or clumps that would otherwise be available for tribo-charging by the carrier particles , the amount of toner which is available for tribo-charging by the carrier particles and development of the charge image is reduced. As a result, development speed or efficiency is decreased, as is toner image density, because an adequate amount of toner particles cannot be supplied to the development zone at a rate fast enough to enable complete image development. This is unfortunate because in order to produce copies of very high resolution, it is necessary to use toner particles that have a very small particle size, i.e., less than about 8 micrometers. (Particle size herein refers to mean volume weighted diameter as measured by conventional diameter measuring devices such as a Coulter Multisizer, sold by Coultor, Inc. Mean volume weighted diameter is the sum of the mass of each particle times the diameter of a spherical particle of equal mass and density, divided by total particle mass).
Accordingly, it would be highly desirable to be able to provide hard ferrite magnetic materials for use as carrier particles, such as the aforedescribed barium, strontium, and lead ferrites having the formula MO.6Fe.sub.2 O.sub.3, where M is barium, strontium or lead, which not only possess the required magnetic properties necessary for providing high speed development and high copy image quality when used in developer compositions comprising such carrier particles and oppositely charged toner particles having particle sizes of approximately 8 micrometers or greater, but which also possess the necessary properties required to provide such high speed development and high copy image quality, when utilized in developer compositions comprising oppositely charged toner particles having particle sizes of less than about 8 micrometers. The present invention provides such carrier particles.